The subject invention relates to a new and improved light pen for use with graphic displays. More particularly, a light pen is disclosed having a new and improved optical design for use in conjunction with unique electrical circuitry, which provides high resolution and substantially reduces the effects of jitter.
In the prior art, light pens have been used as a pointer device in association with computer controlled, cathoderay tube video displays. It is an object about the size and shape of a fountain pen and includes a means for sensing light and a means for converting this light into an electrical pulse. The pen is held in the hand and pointed at some portion of a picture, symbol or the like being displayed on the screen of the cathode-ray tube. When the electron beam which is tracing the image causes a portion of the screen next to the point of the pen to light up, the pen senses this light and generates an electrical pulse which serves as a computer interrupt signal. Typically, the computer circuitry responds to the interrupt signal by reading an address counter having data which tracks the position of the cathode ray at that particular moment. The address in the counter corresponds to the location of the light pen.
As discussed above, when the photodetector of the light pen is actuated, it generates an interrupt signal which typically causes the computer to read a counter having an address corresponding to the location of the cathode ray. Theoretically, if the interrupt signal were generated simultaneously upon the excitation of the phosphor of the pixel, the location of the light pen could be accurately determined. However, in practice, this result has been difficult to achieve. More particularly, the light rays from an excited pixel that are received by the light pen cause a pulse to be generated having an amplitude which varies according to the intensity of the light received. For example, if the light pen is positioned directly over the desired pixel, the intensity will be maximized and the amplitude of the pulse which is generated by the photodetector will be fairly high. In contrast, if the light pen is not directly aligned with a pixel, the intensity of the light rays received will be reduced such that a pulse having a relatively smaller amplitude will be generated.
While the amplitude of a pulse varies with intensity, the "rise time" of all pulses is constant. Rise time is defined as the length of time it takes for the amplitude of a pulse to rise from 10 per cent of maximum to 90 per cent. The rise time of a system is governed by factors such as the tracking speed of the video beam and the type of phosphor used. The problems of detection occur because pulses having the same rise time but different amplitudes will have different slopes. For example, since a large amplitude pulse will reach a maximum value in the same time period as a smaller pulse, the slope of the larger pulse must be greater. Measurement uncertainties arise because pulses having different slopes will exceed detection threshold limits at different times.
Light pens are generally provided with discrimination circuitry to determine if a pulse generated by a photodetector validly represents an excited pixel. Typically, each pulse is compared to a threshold voltage level to determine if the pulse is valid. In order to generate accurate position data, the time between the start of a pulse and the point at which it crosses the threshold level must be constant for all pulses. However, as mentioned above, the time it takes an incoming pulse to exceed the threshold level will vary with the amplitude, which is in turn dependent upon the intensity of the light received by the photodetector. Accordingly, for an aligned pixel, where the amplitude of the pulse is large and the slope is great, the interrupt signal will be generated fairly quickly. However, if the light pen is not directly aligned with the pixel, a pulse having a smaller amplitude and a shallower slope is produced such that the interrupt signal will be generated at a later time.
This timing uncertainty is the cause of jitter. More particularly, the interrupt signal generated by the light pen causes a counter to be read having an address corresponding to the position of the light pen. However, the time the interrupt signal is generated is a function not only of the light pen position, but of the intensity of the light rays received. For example, since a non-aligned pixel will produce a slower rising pulse that exceeds the threshold level at a later time, the address counter will advance beyond the point which would occur with an aligned pixel. Thus, a slight displacement of the light pen can cause the computer to generate a substantially different address.
Jitter may occur even if the light pen is held stationary relative to the video screen. More particularly, the scanning position of the cathode ray includes a predictable amount of error such that in one sweep the ray might be directly aligned with the light pen while on the next sweep it might be slightly off alignment. Accordingly, even if the light pen is held stationary, the interrupt signals used to produce the location data will vary depending upon the alignment of the excited pixel.
In many prior art applications, this jitter effect is not a major disadvantage. For example, light pens are frequently used merely to point to an area on the video screen. Accordingly, precise data regarding the position of the light pen is not necessary since a location determination anywhere within the field was sufficient. However, in other applications, there is a need for a more accurate light pen. For example, if the light pen were to be used for real time drawing, the precise location of each light pen hit must be determined in order to define continuous line segments.
One example of the prior art circuitry utilized to provide more accurate location data can be found in U.S. Pat. No. 3,512,037 issued May 12, 1970 to Eckert et al. The Eckert patent discloses a fairly complex two step scanning system wherein a first scan is used to obtain the gross position data. Thereafter, a second scan is used to disclose horizontal and vertical tangent points. As can be appreciated this two step scan system is complex and difficult to use. More importantly, it requires that the cathode-ray tube perform special searching techniques. Therefore, this method could not be used where the cathode-ray tube was operating in a standard fashion. Accordingly, it would be desirable to provide new and improved discrimination circuitry that is capable of providing accurate position data of the light pen.